1. Field of the Invention
The present invention relates to semiconductor lasers that emit light at visible wavelengths, and more particularly, to Vertical-Cavity Surface-Emitting Lasers (VCSELs) that produce N-frequencies of visible light in a single cavity by altering the optical length of the cavity through the use of a Quantum Well Mirror (QWM) replacing one of the Distributed Bragg Reflectors (DBRs) typically found in a VCSEL.
2. Description of the Prior Art
Semiconductor lasers, such as VCSELs, emit at a visible wavelength in the range of about 400 nm to 700 nm and are of particular interest for applications, such as optical scanning, image display, laser printing and xerography, optical data storage and readout, and plastic-fiber-base optical communications. The VCSELs are known in the art, some of which are described in U.S. Pat. Nos. 5,557,627 and 5,719,892, both of which are herein incorporated by reference. The prior art VCSEL devices commonly are comprised of a gain medium and at least two (DBRs) comprising alternate layers of semiconductive material such as GaAlAs/AlAs. It is desired to have a tunable VCSEL that produces N-light frequencies by adjusting the optical cavity length by applying an electric field to quantum wells devices, in the QWM, changing the refractive index of the well material. It is also desirable to have N of these structures fabricated into an array. In this manner N identical or different frequencies can be produced.
The frequency spectrum of the VCSEL devices is dependent, in part, on the optical cavity length of the laser cavity which, in turn, is dependent on the index of refraction of the alternating layers of the DBRs. One process known in the prior art is to have one VCSEL device produce multiple visible light frequencies in the spectrum by using the oxygenation of a layer or layers in the DBR to change the index of refraction. For example, if a multilayered DBR structure in which one of the alternating layers is composed in part of Al is fabricated and oxygenated after fabrication by introducing oxygen into the deposition chamber, a change in the refractive index of the layer containing Al will occur. The oxygen will migrate into the Al layer from the edge of the structure creating an index of refraction that changes its magnitude in a parabolic manner. This parabolic shape of the index of refraction will produce different visible light frequencies along the length of the AlAs layer. A second method for providing for multiple light frequencies developed by VCSEL devices is the heating of certain areas of the substrate of the DBR during the deposition thereof which will, in turn, creates different indices of refraction in the area being heated and hence creates different light frequencies to be developed by the VCSEL. These procedures are relatively complicated and it is desired to provide for a VCSEL, which can tune to N different light frequencies in a relatively simple manner.
It is a primary object of the present invention to provide for a VCSEL device that is tunable to N different light frequencies.
It is another object of the present invention to provide for a VCSEL device, wherein the fundamental frequency of its associated cavity can be established by the selection of a particular material composition.
It is a further object of the present invention to provide for a VCSEL array whose multiple light frequencies may be easily tuned and modulated.
It is a further object of the present invention to provide for a VCSEL device that may be fabricated of different semiconductor materials that provide a desired bandgap for fundamental light frequencies of interest.
In one embodiment, a tunable multi-frequency VCSEL is provided comprising a first DBR disposed on the surface of a substrate; a first electrode disposed on the DBR; a tuning region composed of quantum wells disposed on the first electrode; and a second electrode disposed on the tuning region. The VCSEL further preferably comprises an active region disposed on the second electrode and a second DBR disposed on the active region.
In another embodiment, an array of tunable multi-frequency VCSELs is fabricated on a substrate each of which has its own external connection to an external power supply. The array can provide N independent identical or different frequencies.